Just know (6)
I believe the answer is:
intuit
'just know' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for intuit that I've seen before include "work things out instinctively" , "Perform (role) in exaggerated manner" , "Understand by instinct" , "Understand instinctively" , "instinctive knowledge" .)